Adequate treatment and prognosis of heart failure patients generally relies on periodic monitoring of characteristics indicative of heart failure. Such characteristics may include cardiac pressure, cardiac volume, ejection fraction and blood flow. In turn, the characteristics may help a care provider to classify a patient's condition, for example, according to the four class scheme of the New York Heart Association (NYHA): Class I—patients with no limitation of activities; they suffer no symptoms from ordinary activities; Class II—patients with slight, mild limitation of activity; they are comfortable with rest or with mild exertion; Class III—patients with marked limitation of activity; they are comfortable only at rest; and Class IV—patients who should be at complete rest, confined to bed or chair; any physical activity brings on discomfort and symptoms occur at rest. Proper treatment of heart failure often relies on assessment of a patient's classification, see, e.g., Shamsham and Mitchell, “Essentials of the diagnosis of heart failure,” Am. Fam. Phys., Mar. 1, 2000 (pp. 1319-1330). For example, Shamsham and Mitchell present an algorithm for diastolic dysfunction and systolic dysfunction that references the NYHA classes.
In general, heart failure can perhaps be arrested, even temporarily remitted, but never cured due to the nature of its most common causes (coronary artery disease leading to myocardial infarction leading to permanently destroyed or damaged myocardial substrate). Therefore, a patient with heart failure requires careful and attentive care and disease management, particularly as the disease state progresses. Once fitted with an implantable pacemaker or intercardiac defibrillator device (ICD), patient monitoring becomes more important and typically more frequent.
Cardiac pressure measurement and echocardiograph techniques can reliably monitor heart failure. However, both techniques require specialized equipment. In vivo measurement of cardiac pressure typically requires use of a catheter lead with a pressure sensitive sensor and echocardiograph techniques cannot be achieved in vivo and rely on external equipment. Indeed, echocardiograph equipment usually requires an in-office consultation. An increase in frequency of monitoring can thus have a tremendous impact on a patient's quality of life. While cardiac pressure measurement may not require in-office consultation, it may require a patient to undergo surgical procedure to implant specialized equipment.
A need exists for techniques for monitoring characteristics of heart failure that have a lesser impact on the patient and her lifestyle. In particular, a need exists for techniques that can be implemented through use of existing, implanted equipment. Various exemplary mechanisms described herein aim to satisfy these and/or other needs.